This new application focuses on the determination of the mechanism by which loss of UBE3A leads to synaptic dysfunction. Based on extensive prior findings, our overall hypothesis is that UBE3A operates to regulate levels of substrate proteins important for neural development, and that disruption of UBE3A activity leads to inappropriately high levels of these substrates, causing the neuronal dysfunctions observed in Angelman Syndrome (AS). The proposed studies are designed to provide improved understanding of the link between aberrations of UBE3A signaling and AS phenotypes. As a long-term goal, our findings will aid in the development of novel approaches for pharmacological manipulation of mis-regulated UBE3A substrates, resulting in new and innovative approaches for the treatment of AS, and possibly other non-syndromic autistic disorders. Specifically: Aim 1. To determine the contribution of elevated Ephexin5 levels to AS- associated phenotypes, to test the hypothesis that elevated Ephexin5 expression in Ube3a mutant mice result in AS-associated synaptic and behavioral phenotypes; Aim 2. To determine the molecular mechanisms controlling UBE3A-mediated Ephexin5 ubiquitination, to test the hypothesis that distinct protein sequences on Ephexin5 and additional cofactors spatially and temporally control UBE3A mediated ubiquitination of Ephexin5; Aim 3. To identify neural substrates of UBE3A, to test the hypothesis that additional UBE3A substrates participate in the etiology of AS.